This invention relates to a continuously variable, traction drive transmissions and, more particularly, it concerns an improved multi-range transmission system of the type in which the input and output torques of a continuously variable transmission are combined as system output torque through epicyclic gearing in a manner to optimize system efficiency and speed ratio range in a compact system configuration.
U.S. Pat. No. 4,630,494 discloses several embodiments of a nutational traction drive transmission representative of a broader class of transmissions in which three frame-supported working bodies operate to transmit a mechanical power input to a rotatable output at infinitely variable output/input speed ratios within the design range of the transmission. The three working bodies have been termed, respectively, an "alpha body" which is supported by the transmission frame to be concentric with a first or primary transmission axis, a "beta body" which is supported by the alpha body to be concentric with a second axis inclined with respect to and intersecting the first axis at a point of axes intersection, and an "omega body" carried by the frame to be concentric also with the first axis. The characterization of these transmissions as "nutational" is based principally on a heretofore preferred mode of transmission operation in which the alpha body is driven in rotation about its axis to carry the beta body in a nutational path as a result of the second transmission axis or the axis of the beta body being inclined with respect to the rotational axis of the alpha body. Another unique feature of such transmissions is that the beta body, which is usually biconical in configuration, makes rolling friction contact with the omega body or omega body members at two points of rolling friction contact on opposite sides of either transmission axis so that the combined torque transmitted by rolling friction through the single beta body is reduced by one half at each of the contact points. Speed ratio variation is achieved by varying radius ratio of the rolling surfaces on the respective beta and omega bodies.
The transmission embodiments disclosed in the afore-mentioned U.S. Pat. No. 4,630,494 represent a departure from prior transmissions of the same general class as a result of orienting the beta and omega bodies in a generally radial configuration. As a result of this orientation, the radius ratio range available to the rolling beta and omega surfaces for a transmission of comparable size is increased substantially over prior embodiments where the beta body axis intersected the primary transmission axis at a relatively small acute angle and where the omega body was represented by a pair of internal rings shiftable axially along the conical surfaces of the beta body. Although the generally biconical configuration of the beta body remained the same as prior designs, the omega body of the more recent design takes the configuration of a pair of plate members spaced on opposite sides of the beta body and of the major portion of a disk-like alpha body. In all illustrated embodiments of U.S. Pat. No. 4,630,494, the nutational movement of the beta body was retained as a result of driving the alpha body in rotation and retaining the omega plates against rotation as the reaction body.
The heretofore preferred mode of operating such transmissions by driving the alpha body so that the beta body is carried in nutation during operation has many advantages. For example, when operating in this mode, the major range of transmission speed ratios are speed reducing; that is, ratios in which the output speed is lower than the input speed. Another major advantage is that the nutational movement of the beta body, coupled with rotation about its own axis, develops gyroscopic precessional forces which act in a direction to augment or increase the normal rolling friction force under which it is held against the omega body members. As a result, bearing load requirements to maintain the rolling friction contact are reduced. On the other hand, the same gyroscopic precessional forces result in a dynamic imbalance of the moving transmission components. While the dynamic imbalance is acceptable for relatively low speed operation or with equipment which is capable of handling the resulting vibrations, vibrations resulting from the nutational movement must be compensated for by counterbalancing in applications, such as automotive drive trains, where vibration-free operation is essential. In addition, the physical organization of the omega body members, particularly the plate form omega members of the more recent radially oriented transmission, present a problem to the attainment of a practical transmission embodiment where the omega plates could rotate as a unit as would be required to avoid the beta body nutation.
In addition to the prior art relating to continuously variable traction drive transmissions, as represented by the afore-mentioned U.S. Pat. No. 4,630,494 and the patent references referred to and cited therein, it is well known in the continuously variable transmission art generally that the speed ratio range of a continuously variable transmission unit can be enhanced significantly by incorporating the unit in a system by which system input is combined with the output of the unit in epicyclic gearing. This is conventionally achieved by driving one of the three components of a planetary gear set directly with the transmission system input, driving the continuously variable unit also directly with system input, driving another of the three planetary gear components with the output of the variable speed unit and taking the system output from the third component of the epicyclic or planetary set. In this manner, operation of the continuously variable transmission unit can be made to provide a system output which varies both in direction of rotation and through an enhanced range of speed ratios in both directions of output rotation. Also, such systems are often equipped with controls to effect spaced or adjacent speed ratio ranges between which the system may be shifted to effect more than one mode of system operation.
In prior transmission systems which combine a continuously variable unit with epicyclic gearing and provide more than one mode of operation, however, one mode, typically providing a low or low/reverse range of system-output/system-input speed ratios and requiring a large amount of power recirculation through the continuously variable unit, is used with another relatively high speed ratio or principal operating mode in which full output power is supplied by the output of the continuously variable unit. The relatively high speed ratio mode can be characterized as the principal operating mode, particularly in applications like automotive drive trains, where this mode is used for a substantially greater period of operating time during the life of the system than the relatively low or low/reverse mode of operation. Although such systems having two, preferably contiguous, continuously variable speed ratio ranges are effective to provide a desired range of continuously variable speed ratios, the power transmission requirements of the continuously variable unit in both modes of operation imposes the total of unit inefficiencies on the system in both modes of operation. As a result, the power transmitting efficiency of the system is compromised. Also, the transmission of full power at all times by the unit, the least durable of components in the power train, shortens the life of the system to an extent that use of this class of transmissions has not been acceptable to the automotive industry, for example.